Citation

Pearson, L. K. (2007). The nature, composition and distribution of sediment in Lake Rotorua, New Zealand (Thesis, Master of Science (MSc)). The University of Waikato, Hamilton, New Zealand. Retrieved from http://hdl.handle.net/10289/2333

Abstract

Lake Rotorua has become increasingly eutrophic over the past 2 to 3 decades. The sediments of the lake have been shown to exert an important influence on this eutrophication process. Chemistry of the sediments has been studied to determine the nature, composition and distribution of elements, through a 1.5 year coring programme. A geophysical survey together with sub-bottom profiling has provided stratigraphic information related to the bathymetry of the lake.

Lake Rotorua has two types of sediments: coarse, dense (density c. 0.5 g/cm3) sediments comprised of clastic erosion products and coarse rhyolitic airfall components covering approximately 60% of the lake area; and fine, low-density (approximatly 0.02 g/cm3) diatomaceous ooze that covers the remaining 40% of the lake, accumulated from deposition of biota, predominantly diatom frustules of Aulacoseira granulata. The sediment contains a record of volcanic eruptions, with the Tarawera Tephra typically found 0.5 m below the sediment water interface and Kaharoa Tephra typically between 2 to 3 m depth, in water depths of 10-15 m.

Phosphorus concentration in Lake Rotorua sediments decreases with sediment depth. In the centre of the lake phosphorus concentrations in the top 2 cm can exceed 2500 g/tonne and decline to 800 g/tonne at 20 cm depth. Accumulation rate of phosphorus in the sediment based on the nutrient budget is approximatly 29.6 t/yr. Iron and manganese concentrations in the sediment depend on the availability of the element and the sedimentation rate of diatom frustules, and are controlled by the redox conditions in the sediment. The average concentration of iron and manganese in the sediment is approximately 8000 g/tonne and between 300 and 400 g/tonne, respectively. Iron accumulates in the sediment at a rate of 385 t/yr and manganese at 17.9 t/yr. Maximum concentrations of arsenic in the sediment are 250 g/tonne but are generally between 50-100 g/tonne, depending on the water depth. Lead concentrations are typically below 15 g/tonne. Sediment concentrations of both arsenic and lead are highly correlated with iron and manganese concentrations in the sediment and mimic the respective concentration profiles. Arsenic and lead accumulate in the sediment at a rate of 3.71 and 0.49 t/yr, respectively. All elements show a peak in concentration in the tephra layers.

The bathymetry of Lake Rotorua is dominated by a curved depression extending from Sulphur Point and almost reaching the Ohau Channel. This depression is probably a structural feature likely associated with the collapse of the caldera, but could be an ancient drainage channel. A series of conical depressions clustered to the north of Sulphur Point and to the east of Mokoia Island are likely to be hydrothermal explosion creators. In the north in the lake at water depths less than 10 m, a series of near-shore terraces are preserved in the sediment.

Sub-bottom echo-sounding shows no return of sonic and seismic signals from most of the lake floor, indicating total absorption by the methane gas-filled sediment. In the shallow lake margin environments, generally less than 10 m water depth, gas is absent and a detailed stratigraphy of multiple reflectors from tephra layers was observed with sub-bottom profiles. The basin sediments of Lake Rotorua are significantly pockmarked, with deep, circular flat-bottomed depressions c. 20-60 m diameter and 0.5-6 m depth. The pockmarks are located on the lake floor in areas where the sediment is saturated with gas.